Taxon Attribute Profiles

photo: Jim Parke,
Healesville Sanctuary

Hydromys chrysogaster

Water-rat

Taxonomy and Ecology

Life Form/Morphology

Hydromys chrysogaster is a distinctive rodent specialised for
an aquatic existence. It is a relatively large murid (bodylength, not
including tail, up to 40 cm) with broad partially-webbed hind-feet, water-repellent
fur, and abundant whiskers. The body is streamlined, with small ears and
eyes, and colour is variable, ranging from slate grey to nearly black
dorsally and from white to orange ventrally. The tail is thick and usually
tipped white. The skull of H. chrysogaster is large and elongated
and there are only two molars on either side of the upper and lower jaws,
a feature shared with the False Water-rat Xeromysmeroides
(Watts and Aslin 1981). More than fifteen synonyms are available for this
highly variable species (Flannery 1995), the validity of most of which,
even at the subspecific level, is questionable.

click to enlarge map

Distribution

H. chrysogaster is found in Australia, New Guinea, and a number
of adjacent islands. This species is widely distributed in eastern Australia,
occurring west to the Kimberleys along the northern coast and west to
Eyre Peninsula on the southern coast. Its range is more restricted inland,
where is found as far west as northeastern South Australia. Isolated populations
occur in southwestern Western Australia and on a number of offshore islands,
including Tasmania and Barrow, Bernier, and Dorre Islands (Olsen 1983).
Inland populations, often associated with temporary water, can be highly
unstable; H. chrysogaster is subject to heat stress and captive
animals are unable to survive high temperatures without large amounts
of water (Watts and Aslin 1981).

Habitat

The Water-rat generally occurs in permanent fresh or brackish water,
although it can also be found in marine environments, including coastal
mangroves in New Guinea (Flannery 1995). The species occupies a wide variety
of freshwater habitats, from subalpine streams and other inland waterways
to lakes, swamps, and farm dams. Populations may be abundant in drainage
swamps, although the Water-rat seems to be much less common along river
channels proper. H. chrysogaster is able to persist in urban areas
and may be one of the few native species to have benefitted, at least
in some areas, from human activity.

Role in Community

H. chrysogaster is a largely carnivorous rodent: crustaceans,
aquatic insects, and fish form the bulk of its diet. Among insects, water
beetles (Dytiscidae) and water bugs (Hemiptera) are of primary importance,
and nymphs of damselflies and dragonflies (Odonata) can be seasonally
important items. Birds, mammals, frogs, reptiles, mussels, spiders, and
plants are also occasionally taken, with plants more commonly consumed
in winter or during periods of limited resources (Woollard et al. 1978;
Harris 1978).

The Water-rat typically forages close to the shoreline, restricting its
movements to shallow water (up to 2 m in depth). In sufficiently shallow
areas, it wades through the water in search of aquatic prey, and it dives
in areas of greater depth (Watts and Aslin 1981). Prey is often taken
to a favourite feeding platform, such as a log, rock, or stump, located
close to the water, where remains of its food are left.

Reproduction and Establishment

Reproduction

Although breeding can occur throughout the year, young of H. chrysogaster
are typically born from September through January. A litter typically
consists of 3-4 young, but litter size is variable and as many as seven
have been recorded. Up to five litters may be produced in years of abundant
food and water, whereas breeding is irregular and smaller litters are
produced in drier years. Social factors also affect the timing of breeding
and age at first breeding. Nesting occurs in burrows in banks of lakes,
streams, and other bodies of water (Olsen 1995).

photo: Jim Parke,
Healesville Sanctuary

Dispersability

H. chrysogaster is a generalist species and shares the good dispersal
capabilities often associated with this life history. It is known to forage
on land and may move considerable distances when doing so. Water-rats
undertake regular movements along shorelines, where their tracks and runs
may be readily seen, and also follow regular routes when crossing bodies
of water (Harris 1978). H. chrysogaster is mainly nocturnal, although
it differs from most Australian rodents in being partially diurnal. It
is most active in the hours following sunset, but may also be found swimming
or foraging during daylight in the early morning or early evening. The
Water-rat is territorial and may be quite aggressive when populations
are at high density.

Juvenile Period

Young are suckled for approximately four weeks and remain with the female
for up to an additional four weeks. Breeding generally does not begin
until eight months of age, although females can become sexually mature
at four months and have been known to breed in the season of their birth
(Olsen 1995).

Hydrology and Salinity

Salinity

Substantial declines of H. chrysogaster have been noted in southwestern
Western Australia and along inland waterways affected by salinity and
degradation (Lee 1995).

Flooding Regimes

The Water-rat prefers slower-moving waters to faster flowing river channels,
and restricted flooding has some potential to affect this species.

Conservation Status

As a species H. chrysogaster is of least conservation concern,
although Although water-use practices have undoubtedly removed habitat
from H. chrysogaster, its current range is likely similar to that
occupied prior to European settlement (Watts and Aslin 1981). The Water-rat
has been considered a pest in irrigation districts (such as those along
the Murray), where it burrows in channel banks and other water-control
and irrigation structures, causing leakage and sometimes collapse of structures
(McNally 1960). Some sources, however, regard this damage as less substantial
than that caused by freshwater crayfish, whose populations H. chrysogaster
helps to control (e.g., Olsen 1983). Natural predators of the Water-rat
include raptorial birds and cats, and the young are vulnerable to snakes
and large fish.

Use

H. chrysogaster was formerly trapped for its fur, but the Water-rat
is now a protected animal in Australia and populations appear to have
recovered from the effects of hunting.

Summary

The Water-rat occurs in a wide variety of aquatic habitats in the Murray
Darling Basin. It is a good coloniser and can be expected to be a reasonable
indicator of the presence of its largely aquatic prey and the general
quality of the water bodies it typically inhabits.

References

Flannery, T. 1995. Mammals of New Guinea. Reed Books, Chatswood,
NSW.

Harris, W. F. 1978. An ecological study of the Australian Water-rat
(Hydromys chrysogaster: Geoffroy) in southeast Queensland. MSc thesis.
University of Queensland, Brisbane.